Science & Measurement

The world around you is made of matter - anything that has mass and occupies space. In chemistry, we study how this matter behaves and changes. All materials on Earth are composed of about 100 different elements, which are the simplest forms of matter with distinct properties that cannot be broken down further.

When you look at pure substances, you're dealing with either elements or compounds. An atom is the smallest amount of an element that still maintains that element's characteristics. When atoms form chemical bonds with other atoms, they create compounds - substances with their own unique properties and definite compositions.

The key difference between elements and compounds is that compounds always contain the same elements in the same proportions, regardless of their source. This consistency is what makes chemistry predictable and allows scientists to identify substances reliably.

Remember this! Pure substances (elements and compounds) cannot be separated by physical means, while mixtures can. This distinction is crucial for understanding chemical reactions.

Elements, Compounds, & Mixtures

When two or more pure substances are physically combined without chemical bonding, they form a mixture. Unlike compounds, mixtures don't have definite compositions and can be separated using physical methods.

Mixtures come in two main varieties: heterogeneous and homogeneous. In heterogeneous mixtures, you can visually distinguish the different components (like oil and water). In homogeneous mixtures, the components are evenly distributed and appear uniform.

Solutions are homogeneous mixtures where one substance is dissolved in another. The most common type is an aqueous solution, where water serves as the major component. Think of dissolving sugar in water - you can't see the sugar particles, but they're distributed throughout.

Properties of Matter

Every substance has a unique set of properties that helps us identify it. These characteristics fall into two main categories: physical and chemical properties.

Physical properties can be observed or measured without changing the substance's identity. These include length, mass, volume, color, and density. When you describe how water looks clear or how metal feels cold, you're noting physical properties.

Chemical properties only become evident during chemical reactions. Flammability (ability to burn) is a classic example - you only know if something is flammable when you try to ignite it. Chemical properties tell us how a substance might change into something new.

Quick tip: Remember the difference between extensive and intensive properties! Extensive properties (like mass) depend on sample size, while intensive properties (like density) remain constant regardless of how much of the substance you have.

Changing Matter

When matter changes, it undergoes either a physical or chemical transformation. During a physical change, the appearance might change, but the chemical composition stays the same - like ice melting into water.

Chemical changes involve a complete transformation where the original substance (the reactant) becomes a new substance (the product). These changes occur when chemical bonds break or form. Every chemical reaction involves energy changes - when bonds break, energy is absorbed; when bonds form, energy is released.

Understanding these energy dynamics helps explain why some reactions feel cold and others produce heat. For example, when you observe a substance losing mass after heating, it suggests a decomposition reaction where part of the compound has escaped as gas.

When a substance gains mass after reacting with air, it has combined with a gas (likely oxygen) to form a new compound. These observations give chemists clues about what's happening at the molecular level.

Matter & Energy

Mass measures how much matter exists in an object. It's directly proportional to weight, though weight can vary with gravitational force while mass remains constant regardless of location.

Energy, the capacity to do work, follows a fundamental principle: it cannot be created or destroyed, only converted from one form to another. This concept, known as conservation of energy, parallels the Law of Conservation of Mass, which states that the total mass present before and after any chemical reaction or physical change remains equal.

Science progresses through several levels of understanding. Hypotheses are initial explanations based on observations that must be tested. Theories are broader explanations that connect many observations and have been extensively verified. Scientific laws are statements of phenomena that are always observed to be true.

Think about it: The scientific process isn't linear! Scientists develop hypotheses based on observations, test them rigorously, and sometimes these grow into comprehensive theories that explain why scientific laws exist.

SI Base Units

Measurements in chemistry rely on the International System of Units (SI), which provides standard units for seven base quantities. These fundamental measurements include the second (time), meter (length), kilogram (mass), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).

Every other measurement in science combines these base units in some way. For instance, volume is measured in cubic meters (m³), and speed is measured in meters per second $m/s$.

To handle very large or very small measurements, scientists use SI prefixes. These modifiers change the value of the base unit by powers of ten. The prefix "kilo-" multiplies a unit by 1,000, while "centi-" divides it by 100. This system allows scientists to express measurements from the astronomical to the microscopic using the same basic units.

Measurement Precision

In the scientific world, how you measure matters just as much as what you measure. When scientists repeat experiments, they're looking for consistency in their results.

Accuracy refers to how close your measurements are to the true value, while precision describes how close repeated measurements are to each other. You can think of accuracy as hitting the bullseye on a target, while precision is hitting the same spot repeatedly (even if it's not the bullseye).

When calculating results, scientists use significant figures (sigfigs) to indicate the precision of their measurements. Some numbers, however, don't limit sigfigs in calculations. These include exact numbers like definitions (100 cm in 1 m), counting numbers (10 students), and integers in formulas $d=2r$.

Pro tip: When reporting your lab results, pay attention to both accuracy and precision. A measurement can be precise (consistent) without being accurate (correct), or accurate without being precise!